Fuser control circuit

ABSTRACT

A FUSER CONTROL CIRCUIT FOR USE IN AN ELECTRONIC PRINTER HAVING APPARATUS TO SENSE SIGNALS INDICATIVE OF PAPER OR RECORDING WEB SPEED. OTHER APPARATUS RESPONSIVE TO THE SENSED SIGNALS APPLIES POWER TO THE FUSER WHICH IS PROPORTIONAL TO THE PAPER SPEED. THE POWER APPLIED TO THE FUSER HEATER ELEMENT CONTROLS THE TEMPRATURE TO THE FUSER AND THUS, THE HEAT APPLIED TO FIX DEVELOPED LATENT IMAGES ON THE TRAVELLING PAPER.

United States Patent [72] Inventor Charles L. Hopkins 3,041,438 6/1962 Lovegrove 219/492X West Webster, N.Y. $219,794 11/1965 Mindell et al... 219/216 [21] Appl. No. 792,055 3,224,355 12/1965 Thomiszer 250/65. 1 (X) [22] Filed Jan. 17,1969 3,326,692 6/1967 Martino et a1. 219/497 [45] Patented June 28,1971 3,264,496 8/1966 Scholl 317/5X [73] Assignee Xerox Corporation 3,351,829 11/1967 'Qvamstrom..... 318/162X Rochester, N.Y. 3,431,398 3/1969 Gunter Wahle 219/388 3.346.712 10/1967 Seyfried 317/5X [54] FUSER CONTROL CIRCUIT Primary Examiner- Bernard A. Gilheany 8 Claims, 5 Drawing Figs. Assistant Examiner-F. E. Bell Atlarneys Paul M. Enlow, Ronald Zibelli, James J. Ralabate, [52] US. Cl 219/50],

2l9/2l6' 219/388. 219/469 328/140 Norman E. Schrader and Laurence A. Wright [51] lnt.Cl 05b H02 [50] held Search 250/651; ABSTRACT: A fuser control circuit for use in an electronic 335/97; 5O 1; printer having apparatus to sense signals indicative of paper or 328/140; 317/5; 318/162 recording web speed. Other apparatus responsive to the sensed signals applies power to the fuser which is proportional [56] References CM to the paper speed. The power applied to the fuser heater ele- UNITED STATES PATENTS ment controls the temperature of the fuser and thus, the heat 2,323,852 2/l 58 Moore 328/140X applied to fix developed latent images on the travelling paper.

DEVELOPING STAT ION //O- 6.3 /08 RECORDING FUSER STATION 111: F1: cg fig'a FL 1PFLOP SYNC clRcUl-r CIRCUIT PULSE PATENTEDJUNZBIQYI 3. 588445 I .SHEET 1 OF 4 /09 DEVELOPING STATION as Y/oa RECORDING FUSER STATION F F}: FUSER FLIP-FLOP SYNC CONTROL CIRCUIT cmcurr PULSE FIG. 1

INVI'IN'IOR. CHARLES L. HOPKINS ATTORNEY PATENTHMunzsm: 3 5 5 SHEET 3 BF 4 1 c1.UPPER LIMIT (YELLOWING) 90- J, 60- FUSER d.LOWER LIMIT VOLTAGE 501 (JUST FUSING) (voLTs) 0 I I I I I I I i PAPER SPEED lN./M|N.

(STEPPED) FIG. 3

FUSER CONTROL CIRCUIT BACKGROUND OF THE INVENTION This invention relates to improvements in control circuits. More particularly, the invention relates to a control circuit for use in controlling the temperature of a fuser for fixing images formed on paper sheets or a recording web with fusible powders.

It is desirable in control devices that the response to changed conditions be as immediate as possible. It is known in the prior art as taught by U. S. Pat. No. 2,586,484 to control the heating element of a fuser by a bimetallic sensing means or by a thermistor as taught by U. S. Pat. No. 3,327,096. The bimetallic elements are mechanical devices which flex in response to heat changes to intermittently open and close switch contacts in the electric control circuit of the heater to maintain a predetermined temperature level. Ordinarily, bimetallic elements of conventional design are relatively slow to respond and because of repeated mechanical switching-actions, have a limited operative life. The contacts erode with use and accumulate dirt which makes operation unreliable. The thermistor is a solid semiconductor having a negative or a positive temperature coefficient of resistance. It has the disadvantage in control circuits of being influenced not only by the heater element but also by ambient conditions. Another disadvantage of the thermistor is that it must be physically close to the heater element to sense the temperature changes. Further, because of their mass, an undesirable thermal leg is introduced into the thermistor response time. Although faster responding thermistors are known they do not lend themselves to practical applications for mechanical reasons, such as handling, installation and mounting. While the bimetals and thermistors have performed satisfactorily in some instances, they have not always been satisfactory in maintaining the fast and reliable response necessary in fuser usages.

In electronic printing or photography wherein latent images are electrostatically formed on the recording web of paper sheet the latent images are developed and fixed by placing finely developed fusible powders in contact with the electrostatic image and applying heat to fuse the powder on to the recording web or paper sheet to develop visible images. If the temperature of the fuser heater is below the fusing requirement the powder particles may not fuse to the recording web or paper sheet. On the other hand, if the temperature of the fuser is too high, the recording web or paper sheet may be yellowed or charred.

In some recording applications such as is the concern of this invention, the recording web or paper speed varies as it passes the fusing station. One such application is in facsimile machines wherein the recording web at a recording station is stepped past a recording unit, such as an electrographic recording stylus, at a faster rate when no information is to be recorded than when the unit is recording the transmitted information on a line-by-line basis. The most economic operation of the facsimile unit occurs when standard DDD telephone lines are used during nonbusiness hours. At this time the receiving station is generally unattended and a continuous recording web is practical. As a result, during the interdocument white space, for example, the web may be stepped at a high rate past the recording unit and at the fusing station the temperature must respond to this changed condition.

Therefore, consideration must be given to the speedof the paper as it passes through the fuser. That is, if the paper is travelling slowly past the fuser less intensity of heat is required to fix the developed latent image on the paper. While on the other hand, when the paper is travelling rapidly past the fuser a higher intensity of heat is required since the paper is in the presence of the fuser for a shorter period of time.

The present invention is adapted to control the temperature of a fuser heater element within limits prescribed for electrostatic recording purposes. The response to a call for a temperature change is very rapid, in the order of milliseconds.

Moreover, the invention eliminates the need to overcome the inertia of mechanical devices such as bimetals. Further, the invention eliminates the need for direct heat sensing That is, the invention does not sense the temperature of the heating element per se as thermistors do. On the contrary, the invention maintains critical control of the fuser temperature by detecting the width of pulses applied to the stepping motor that drives the paper. Consequently, the power applied to the fuser heater element is proportional to the paper or recording web speed.

It is also within the scope of the invention to detect the width of pulses which may emanate from the paper feedoperating circuit or pulses from an end of transmission circuit in the recording station.

It is an object of the invention to control the temperature of a fuser wherein the power applied to the heater element is related to or is proportional to the paper speed.

It is a further object of thisinvention to control the response time ofthe fuser heater element by eliminating a heat sensor.

It is a still further object of this invention to provide a fuser control which is unaffected by ambient conditions.

It is yet a still further object of this invention to eliminate the mechanical problems inherent in the useof thermistor sensors.

It is a further object of the invention to provide an electronic thermostatic circuit having an extended life and improved reliability.

These and other objects of the invention are achieved by means ofa control circuit including a timing circuit which detects the width of pulses indicative of recording web or paper speed in a first, second and third mode and which controls the power applied to the fuser heater element in accordance with the recording web or paper speed.

In order to gain a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description of the invention to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a simplified block diagram of an electronic printing system; v

FIGS. 2a and 2b are schematic illustrations of the control circuit;

FIG. 3 is a plot of paper speed vs. voltage; and

FIG. 4 shows the circuit waveforms generated by the circuit of FIGS. 20 and 2b;

A fuser control of the type to be described may be employed in an electronic printing device as shown in block outline in FIG. 1 in which all the components except the fuser control circuit are conventional.

Referring now to FIGS. 1 and 4a, reference numeral designates a block from which a source of sync pulses are obtained to drive a flip-flop circuit 101. The sync data may be obtained from the paper feed-operating circuit, an end of signal apparatus in the recording station or any monitoring means which detects the recording condition. The flip-flop circuit 101 generates variable width pulses (FIG. 4a) which are applied to one of the windings of a stepping motor 103. The same pulses from flip-flop circuit 101 are concurrently applied to fuser control circuit block 102 to furnish data related to paper speed.

While the block diagram of FIG. 1 shows variable width square wave input pulses to the fuser control circuit 102, in lieu of the square wave input signals other signals having the proper repetition rate could be substituted. That is, signals taken directly from sync pulse block 100 may be applied directly to fuser control circuit block 102 and still achieve the desired results of the invention. Therefore, the variable width square wave input to block 102 is taken merely as a convenient signal source although other sources could be furnished. In operation of FIG. 1, the recording web or paper 110 is installed over supply reel 104, idler rollers 105 and 106 and take up reel 107. Upon the application of pulses from flipflop circuit 101 the stepping motor 103 drives recording web 110 past the recording station 108, the developing station 109 and fuser 63. At the recording station 108 images are electrostatically recorded on the recording web. Fusible powders are applied to the web at developing station 109. Subsequently, the images will be fixed by the application of heat on the recording web by fuser 63. Since stepping motor 103 and fuser 63 are controlled by the same source, the amount of heat applied to the recording web is made proportional to paper speed. That is, when the recording web is moving slowly through the fuser less heat is applied to the recording web and conversely more heat is applied when the recording web is moving rapidly through the fuser.

Referring now to FIGS. 1, 2a and 4, the circuit sense the paper speed range or mode by detecting the width of the input pulses. The input pulses (FIG. 4a) may be obtained from flipflop stage 101 that drives one of the windings of stepping motor 103. The waveform of FIG. 4a shows the pulse width during the various modes of operation. The wider the input pulses the slower is the speed of the stepping motor and thus paper or recording web speed. The input pulses are fed into the base of buffer transistor Q11 and are amplified and inverted. The resistors '13 and 14 connected between ground and a +12 v. reference potential form a voltage divider to furnish the bias on the emitter of PNP transistor Q11. The amplified signal (FIG; 4b) developed across load resistor 12 and the -12 v. reference potential is then fed in parallel through resistors 16 and 17, to the bases of NPN transistor Q1 and NPN transistor Q5 respectively. These two transistors form the input circuits of two channelsor paths associated with the lowand high-speed range of the recording medium.

Transistors Q1, Q2, Q3, Q4 as well as silicon-controlled rectifier SCR 44 in the low-speed channel or path are connected to the +12 v. bus through their individual load resistors. Transistors Q5, Q6 and Q7 in the high-speed channel or path are connected to the +12 v. bus in a similar manner.

The description 'will first proceed with a discussion of transistor Q1 and the low-speed mode of operation. The pulse width of the signal from collector resistor 25 of transistor 01 is detected by the RC timing circuit of variable resistor 22 and capacitor 24. When the duration of the negative pulses on the base of transistor 01 is equal to or greater than the time needed to charge capacitor 24 to the critical voltage V, shown in FIG. 40 unijunction transistor Q2 fires since its emitter rises with the charge on capacitor 24. The conduction of transistor 02 discharges capacitor 24 across base resistor 32 providing positive pulses (FIG. 4d) indicative of paper speed determined by the RC timing circuit of variable resistor 22 and capacitor 24. These positive pulses are applied to the cathode of siliconcontrolled rectifier SCR 44 through diode 34, across resistor 36, through capacitor 38 and across resistor 46. The pulses or control signals applied to the cathode of SCR 44 insure that SCR 44 remains nonconducting when the stepping motor is operating in the low-speed range.

The positive voltage from the junction of the anode of SCR 44 and resistor 42 is fed to amplifier, PNP transistor Q3. Transistor Q3 remains off since its base-emitter junction is back biased, permitting capacitor 54 to charge sufficiently through resistor 56 to cause unijunction transistor Q4 to fire and go into oscillation. The signals (FIG. 3h) from transistor Q4 are then coupled to the primary of transformer T1.

The low-speed channel will be inoperative during normal operations when high-speed pulses appear at the base of transistor 01. This is the case because the duration of the high speed pulses is less than the time needed to charge capacitor 24 to the critical voltage V, of FIG. 40. Thus, the low speed channel is effectively bypassed during the high-speed mode of operation.

Referring now to FIG. 2b, the 60 cycle l v. input voltage is connected in series with fuser 63 which contains the fuser heater element. The heater element of fuser 63 may be a resistive coil, a heat-radiating source or the like, the temperature of which is controllable by the fuser control circuit. A bridge diode circuit 64 is connected across the voltage supply and is in series with the fuser as shown. Across the junction of the diodes of the bridge circuit, leads 9] and 92 connect the portion of the control circuit which actuates SCR 89. The bridge circuit 64 and fuser 63 are conventional circuit elements and are well known in the art.

The signals from the primary of transformer T1 are coupled to the secondary of transformer T1 which is shunted by rectifying diode 67 at terminals 1 and 2. The rectified signal is then applied to the gate electrode of silicon-controlled rectifier SCR 69 through resistor 68. The pulses produced by transistor Q4 causes SCR 69 to turn on early in the cycle bridge output voltage to which SCR 69 is referenced via leads 91 and 92.

Zener diode 66 in conjunction with resistors 71 and 72 maintain the desired voltage level across SCR 69 and its associated circuit components. The conduction of SCR 69 produces a voltage change across resistor 73 to turn NPN transistor Q8 off, restricting capacitor 82 to the charging path of variable resistor 83. Zener diode 74 is used to clamp the control circuit voltage to a fixed level. Resistors 76 and 77 in conjunction with diode 78, provide an adjustable pedestalcharging voltage to capacitor 82.

When the voltage on capacitor 82 reaches V, of FIG. 4j unijunction transistor 09 fires and the silicon controlled rectifier SCR 89 conducts (FIG. 4i) for a shorter portion of the 120 cycle bridge output voltage waveform, applying low power (FIG. 41) to the fuser heater element representative of the low paper speed. The low power applied to the fuser heater controls the temperature of the fuser so that the developed latent image is fixed upon the paper which at this time is travelling slowly through the fuser. Variable resistor 83 controls the low power applied to the fuser by regulating the point in time at which V, is reached by capacitor 82.

In the high-speed mode of operation, the circuit associated with NPN transistor Q5 (FIG. 2a) controls the temperature to provide higher intensity of heat for fixing the developed latent images on the paper since the paper is under the influence of the fuser for a shorter period of time. The circuit is also adjusted to prevent yellowing or charring of the paper. Thus, a negative pulse across collector load resistor 23 of transistor 05 is fed to the base of NPN transistor Q6 (FIG. 4ecausing the collector which is connected through resistor 47 to the junction of RC timing network of-variable resistor 49 and capacitor 51 to rise positively. Positive pulses on the collector of transistor OS are shunted to ground by diode 29 so that they never reach transistor Q6. However, negative pulses are passed to transistor 06 via capacitor 27, diode 29, resistor 33 and diode 37. While transistor 06 is nonconducting capacitor 51 will begin to charge (FIG. 4}) to the positive rise across collector resistor 47 of transistor Q6 and in a few cycles will have built up sufficient charge to fire unijunction transistor Q7.

It is well to note here that the wide negative pulses present during the low-speed mode of operation are insufficient to fire transistor Q7 because the negative voltage present on capacitor 39 will have had sufficient time to decay through resistor 31 to V Q6 then turns on and thereby discharges capacitor 51 through resistor 47.

Thus, transistor O7 is prevented from firing during the low speed mode of operation, since capacitor 51 cannot charge to voltage V, (FIG. 4F).

When transistor 07 fires, a pulse or control signal (FIG. 4g) developed across resistor 57 is fed to the gate electrode of SCR 44. SCR 44 turns on and feeds a less positive signal to the base of transistor O3 to turn transistor 03 on. The current through collector resistor 52 of transistor 03 causes transistor 04 to become disabled since the emitter of transistor 04 is close to ground potential. With transistor Q4 nonconducting no signal appears at the gate of SCR 69 (FIG. 2b). SCR 69 is cut off allowing transistor Q8 to conduct. The conduction of transistor 08 permits capacitor 82 to charge up quickly through the paths of variable resistor 83 and transistor Q8 itself and its variable emitter resistor 81. When the junction of the emitter of transistor Q9 and capacitor 82 reaches V, (FIG. 41') transistor Q9 fires, thus turning on SCR 89 (FIG. 4k)

which conducts for a greater portion of the 120 cycle bridge output voltage waveform (FIG. 4i) applying high power to the fuser heater element (FIG. 41). The high power and thus the temperature of the fuser is adjustable by means of resistor 81. As a result of the foregoing, it is seen that the narrow pulses applied to the stepping motor are related to the higher temperature applied to the paper travelling through the fuser.

Referring to the graph of FIG. 3, there are a series of curves representing a plot of paper speed vs. r.m.s. voltage. The curve designated a is the upper limit curve and indicates the point of paper yellowing at a discrete paper speed and heater voltage. Curve d shows a discrete point of paper speed and heater voltage where fusing will just occur. That is, no fusing will occur for any given paper speed if the voltage applied to the heater element falls below curve d. Broken-line curve b, along with broken-line curve 0 represent the safe interval between fusing and yellowing. That is, the paper speed and fuser voltage are adjusted so that the temperature of the fuser never falls in the region of the curves where undesirable effects are likely to take place.

Referring again to the circuit of FIGS. 2a and 2b, it may be adjusted to have two discrete points arbitrarily spaced from one another to discriminate between the speed ranges desired. That is, the timing circuit of variable resistor 22 and capacitor 24 in the low-speed detecting channel and the timing circuit of variable resistors 31 and 49 and capacitor 51 of the high-speed detecting channel are adjusted for the desired setting. Moreover, the channels may be adjusted so that when in the crossover" range or mode the timing is such that the circuit will oscillate alternately between the high-voltage and lowvoltage range. An illustration of this is shown in F l0. 3. An arbitrary crossover" point of 4 inches per minute is chosen for the illustration. The circuit may be adjusted to oscillate between the high and low voltage when in the range 3.5 to 4.5 inches per minute in order to average the power to the fuser when going through or setting at 4 inches per minute.

From the foregoing, it will be understood that the sensed pulses, indicative of the paper speed, control the charging and discharging time of capacitors 24 and 51 which in turn determines whether oscillator transistor Q4 will turn off or on. De-

pending upon the presence of a signal or lack of it from transistor 04, SCR 89 will conduct for a greater or lesser portion of the l cycle bridge output voltage wave. This arrangement provides fast response to paper speed conditions without the disadvantages mentioned above.

I claim:

1. A control circuit for a fuser for controlling the energization of the fuser heater element comprising:

a fuser,

means for transporting a recording medium to said fuser to fuse latent images thereon said transporting means having variable speeds,

means for generating time-varying pulses adapted to drive said transporting means, means responsive to said generating means for sensing signals indicative of the speed of said recording medium,

first means responsive to the duration of said sensed signals in a first paper speed mode for generating a first control signal,

second means responsive to the duration of said sensed signals in a second paper speed mode for generating a second control signal, and

means responsive to either of said control signals for applying power to said fuser heater element for a period related to the duration of said sensed signals.

2. A fuser control circuit comprising a rectifier bridge circuit connectable to a power source, a heater connected in series with said rectifier bridge circuit, a semiconductor-controlled rectifier connected in parallel with said bridge circuit, first and second detecting means responsive to input signals to said control circuit in a first and second mode, said semiconductor-controlled rectifier responsive to said first and second detecting means and conducting for a period representative of the duration said input si nals to supply power to said heater.

3. The apparatus of c aim 2 wherein said first and second detecting means each includes a unijunction transistor and a resistive-capacitive timing circuit.

4. The apparatus of claim 2 comprising a second semiconductorcontrolled rectifier means and an oscillator circuit coupled to said second semiconductor-controlled rectifier and wherein said first and second detecting means are connected to said second semiconductor-controlled rectifier means to control the conduction of said oscillator circuit.

5. The apparatus of claim 3 wherein said first detecting means includes means for preventing said second semiconductor-controlled rectifier from conducting and said second detecting means includes means for permitting said second semiconductor-controlled rectifier to conduct, whereby the oscillation of said oscillator circuit reflects whether said first or said second detecting means controls the power applied to said heater.

6. A fuser temperature control circuit for controlling the energization of a heater element in accordance with paper speed,

a rectifier bridge circuit connectable to a power source,

a heater connected in series with said rectifier bridge cir cuit,

a silicon-controlled rectifier connected in parallel with said bridge circuit,

means for sensing input signals indicative of the paper speed,

first and second detecting means responsive to said sensing means to control the conduction of said silicon-controlled rectifier, said first and second detecting means being responsive to input signals in a first and second speed mode, and

means coupling the output of said detecting means to said silicon-controlled rectifier, said silicon-controlled rectifier conducting for a period representative of the duration of said sensed input signals and applying power to said heater element for said period.

7. In a xerographic reproducing apparatus wherein an electrostatic latent image is formed on a dielectric surface and transferred to a recording web at a recording station and having a developing station at which finely divided toner particles are attracted to the electrostatic latent image and also having a fuser station at which the toner particles are fused to the recording web, a fuser control circuit for controlling the energization of the fuser heater element in accordance with recording web speed comprising:

a source of control signals emanating from said recording station, said signals having a pulse repetition rate related to the recording rate of said recording station,

means for sensing said signals indicative of said recording rate from said recording station,

a first pulse-duration-responsive circuit connected to said sensing means for generating a first control signal when said recording station is operating in a first mode,

a second pulse-duration-responsive circuit connected to said sensing means for generating a second control signal when said recording station is operating in a second mode, and

means responsive to either of said control signals for applying power to the heater element of said fuser related to the operating mode of said recording station.

8. The apparatus of claim 7 including:

means for switching to the output of said first or second pulse-duration-responsive circuits said means including adjustable means for repetitive switching between said first and second pulse-duration-responsive circuits. 

